The present invention relates generally to power amplifiers. More specifically, a radio frequency power amplifier is disclosed.
Power amplifiers are used in a wide range of applications. Many communications systems, particularly wireless communication systems, use power amplifiers to amplify the signal before it is transmitted.
FIG. 1 is a block diagram illustrating a typical radio frequency (RF) power amplifier (PA) design. The voltage needed by the circuit is provided by voltage supply 125. A load resistor 100 is coupled between the supply voltage and the output. Two transistors 105 and 110 are coupled in a cascode configuration, where transistor 110 receives an RF modulated input signal at its gate. The signal is amplified and output at terminal 120 of transistor 105. A DC biasing voltage is applied to the gate of transistor 110 at junction 115. The biasing voltage sets the linear range of the power amplifier.
To amplify an RF input signal with a non-constant envelope using a class A amplifier, the biasing voltage is typically fixed to a relatively high value so that there is a large current flowing through the circuit, enabling the power amplifier to linearly amplify an input signal of relatively high amplitude. Such a high biasing voltage leads to waste of power since a low amplitude input signal needs less DC current than a high amplitude input signal to achieve the desired gain. Moreover, as the amplitude of the input increases, the gain provided by the power amplifier decreases and the output of the higher amplitude input signal becomes compressed, causing non-linearity problems.
There have been attempts to predict the output envelope size and then set the biasing voltage accordingly. This approach can be quite complex, and sometimes introduces stability issues. It would be useful to have a stable power amplifier that has low power consumption and provides good linearity.